Environmental and physical factors controlling the distribution of 137Cs in lake sediments in the Southern Hemisphere: a meta-analysis

For decades the artificial radionuclide ¹³⁷Cs has been used as an independent time marker to ascertain the ²¹⁰Pb chronology to date recent sediments from the Anthropocene period (<150 years). The distribution and depositional timing of man-made fallout radionuclides (FRN) are well constrained in the Northern Hemisphere, where most nuclear weapon test sites were located. The maximum deposition year of 1963 is usually marked by a ¹³⁷Cs peak. Although the major nuclear powers stopped testing in 1963, France continued to test atmospheric nuclear bombs (1966-1974) in French Polynesia in the Pacific (Moruroa and Fangataufa atolls). This later and prolonged period of French bomb testing in the Southern Hemisphere may have resulted in a wider distribution with higher FRN levels in depth profiles of environmental archives, such as lake sediment cores.

To test this hypothesis, a literature review was conducted (n=124), in which ¹³⁷Cs data were collected from lake sediments (including dam reservoirs and lagoons) across the Southern hemisphere. Decay-corrected ¹³⁷Cs activities, ¹³⁷Cs inventories (where available) and parameters of the ¹³⁷Cs profile shape have been reported for many countries and latitudinal bands. In addition, environmental and physical parameters were reported for each lake site. Global parameters influence the atmospheric distribution and deposition of FRN such as the distance from the nuclear test site, the wind distribution (relative to the Intertropical Convergence Zone position), the wind direction (westerlies vs trade winds) and the annual precipitation. Conversely, local scale parameters such as sedimentation rate, catchment to lake area ratio, and maximum elevation difference will influence the depositional processes of FRN in lake sediments. A meta-analysis of these parameters will help to identify parameters that are crucial for understanding the ¹³⁷Cs distribution across the Southern Hemisphere. Based on these results, we selected the new sampling sites, which are likely to reflect mainly FRN atmospheric input, for further reconstruction of fallout radionuclide chronologies in the Southern Hemisphere.